A group of guys, some wealthy, some smart, and some that were neither, dreamed of a government elected by the citizens it governed, and then refreshed periodically with term limits and newly elected representatives. To protect the new government from its self they divided it into three parts; the executive branch (President), the congress (Senate and House of Representatives), and the judicial (Supreme Court). Each of these divisions was to oversee the other two and was empowered to override. The guys made a few mistakes which were corrected as the government grew larger but there have been, in 235+ years, remarkably few corrections.
The original government structure was distilled from previous governments and strongly linked to ancient philosophers, the Judaeo-Christian religion and ethical and moral teachings, and the factual experiences of history. This foundation was negatively influenced by the oppressive demands of the rulers of the countries the guys came from. The primary stick in the mud was King George of England, and the most common complaints about his dictatorial administration centered on unjust taxation, the inability to own property, and the lack of personal freedom. All of these elements affected the design for the new American government.
So, it began in 1776 with the Revoutionary War, continued for about 75 years relatively at peace, and then experienced the largest and most brutal civil war the world had ever seen. In and around the Civil War in 1865 America began making societal changes that effectively influenced it's continued growth. The most well known of these was giving the negro slaves their freedom. Soon after the Civil War our nation began it's industrial revolution. America's population started to move from rural to urban areas as citizens moved towards increasing employment opportunities as factories were established in urban areas. This movement was well underway during the time of the Civil War in 1865 and the turn of the century in 1900.
Europe had a number of difficult problems. Problems with religion, economics, and ambitious leaders. In 1917 America joined it's allies in the 'war to end all wars', the First World War. It was fought in Europe, it became huge, involved many countries, and resulted in an enormous number of military and civilian casualties. Actually, advances in weaponry resulted in a horrific number of deaths and the destruction of entire towns and cities. This became the most devastating war the world had ever endured. Then it ended.
Peace and harmony prevailed for about 60 years before the German, Italian, and Japanese nations (the Axis) attempted to expand their territories. In the mid-1930's they were resisted but successful as they attacked less powerful, sometimes defenseless, and usually smaller nations. America formally joined the war in 1939, a bit after it had begun. We combined with England, France, Canada, Australia, New Zealand and a few other nations (the Allies) to defeat the Axis enemies. And we did defeat them.